1. Field of the Invention
The present invention relates to direct current (DC) offset cancellation, and more particularly to DC offset cancellation in a wireless communication device.
2. Prior Art
Receivers are necessary components of communication links, and are used, for example, in two-way cellular phone communications or wireless local area networks. A simplified block diagram of a typical prior-art wireless receiver 100 is shown in FIG. 1. A signal from an antenna is fed to low-noise amplifier (LNA) 110. LNA 110 is a variable gain amplifier controlled by an automatic gain control (AGC) circuit for amplifying weak signals as necessary without introducing much noise. LNA 110 feeds mixer 120, which mixes down the received high-frequency signal to baseband frequencies (including 0 Hz), by effectively multiplying the received and amplified signal with a local-oscillator (LO) signal produced by an oscillator (not shown) in the receiver. The undesirable signals at very high frequencies produced by the mixing (multiplying) process are filtered out by baseband filter 130. The filtered signal is then amplified by baseband amplifier 140. The gain of baseband amplifier 140 is also made variable through AGC action; the gain being large when the received signal is weak, and small when the received signal is strong. The objective of this operation is to keep the output signal to a well defined power, so that it can be encoded by an analog-to-digital converter, or otherwise used, without undue distortion and noise.
A significant problem encountered in the design of direct-conversion receivers is the generation of a parasitic direct-current (DC) quantity, called DC offset, at the output of mixer 120. One of the main causes of this phenomenon is the parasitic leakage of some of the local oscillator signal to the antenna; this signal is then amplified by LNA 110 and mixes with itself. The result is a twice frequency component which gets filtered out by the baseband filter and a DC component corresponding to the DC offset mentioned above. This zero-frequency component may cause the output of the receiver to reach saturation. The problem is especially severe for standards in which the baseband extends all the way to nearly zero frequencies. A DC offset also occurs when the signals are not in-phase. The DC offset produced will depend on the phase difference between the signals. Such phase difference is unpredictable, as it can vary depending on fabrication tolerances and other factors. Another cause of an unpredictable DC offset is the leakage of a received signal to the local oscillator port. The DC offset due to all of the above factors can pass through baseband filter 130 and then be amplified by baseband amplifier 140, resulting in a large undesirable DC component at the output of system 100. The value of this component can be so large that certain circuits may reach saturation. This includes the output of baseband amplifier 140, resulting in nonlinear operation and thus signal distortion. In addition, this DC component shifts the desired signal at the output, and can lead to saturation or otherwise inadequate operation of a following stage analog-to-digital converter of the wireless receiver.
A wireless receiver operating, for example, in accordance with the IEEE 802.11a standard, uses orthogonal frequency division multiplexing (OFDM). Each frame transmitted has a preamble sequence 200, shown in FIG. 2. The preamble field is composed of ten repetitions of a “short training sequence” 210, used for AGC convergence, diversity selection, timing acquisition and DC offset cancellation in the receiver. The preamble field is further composed of two repetitions of a “long training sequence” 220, used for channel estimation and fine frequency acquisition, preceded by a guard interval 230. A short OFDM training symbol consists of 12 sub-carriers (±4, ±8, ±12, ±16, ±20 and ±24 with 312.5 KHz of spacing for 802.11a, and ±2, ±6, ±10, ±14, ±18 and ±22 with 312.5 KHz of spacing for Hiperlan2). The DC offset cancellation is to be achieved in the time frame of preamble sequence 200.
In view of the limitations of prior art solutions, it would be advantageous to provide an effective means for reducing the DC offset at the output of mixer 120, resulting in only a small DC component at the output of system 100. It would be further advantageous if such DC offset cancellation process did not interfere with the AGC operation. It would be further advantageous if such DC offset cancellation is achieved within the preamble 200 time frame, preceding the actual receipt of data.